The UIC ACE will focus over the next 5 years on the genetics, neurobiology, cognitive and affective processes, and pharmacology of insistence on sameness (IS) in autism spectrum disorders (ASD). A large sample of children with self-reported autism spectrum disorder will be screened by the Assessment Core for further screening by administration of the ADI-R to the parents. Profanes meeting ADI-R criteria for autistic disorder will be recruited for further study if they are also classified by the ADI-R IS items as high (N=150) or low IS (N=100). In addition, high IS subjects will need to score 15 or more on the sum of two IS factors on the RBS-R to avoid floor effects for the pharmacogenetic trial. These 250 subjects will all be included in project I, Genetics of Serotonin in Autism: Neurochemical and Clinical Endophenotypes, along with 225 previously studied subjects and their parents for a total of 475 trios. This project will study 25 serotonin- related genes for association with autism and with IS more specifically. Resequencing of strong candidate genes will be conducted with all of the subjects in the pharmacogenetic project (III) and with the low IS subjects in project II. In addition, the 250 subjects will have serotonin measures collected for analysis with genetic and phenotype measures. In Project II: Translational Studies of Cognitive, Affective and Neurochemical Processes Underlying Insistence on Sameness in Autism, fMRI studies of IS will be conducted on 50 high IS subjects also in Project III, 50 low IS subjects (also in Project I) and 50 control subjects. In addition, rat studies in which parallel behavioral and neurochemical approaches will be used. Project III: The Pharmacogenetics of Treatment for Insistence on Sameness in Autism has been designed to replicate and extend a preliminary study of escitalopram treatment of IS related irritability in ASD. Project IV: Autism-Associated Serotonin Transporter (SERT) Mutations will provide characterization of mutations previously found to be associated with high IS behaviors in subjects with autism. The UIC ACE is an exciting center that brings together experts in a diverse set of disciplines to comprehensively study IS, one of the two cardinal features described by Kanner in 1943.